1. Field of the Invention
This invention relates to a residual capacity meter for detecting the degree of deterioration of a battery, and measuring the residual capacity of the battery taking into account the degree of deterioration.
2. Description of the Prior Art
A motor-driven electric car is known in the art which generates very low pollution. This electric car is powered by batteries, and the state of charge of these batteries has to be measured. The power in the batteries is used to drive the motor, and when the residual capacity in the batteries approaches 0, the car can no longer run. It is therefore necessary to charge the batteries before the residual capacity has fallen below a predetermined value, and to measure this state of charge.
Various types of charge meters have been described in the art. A power integrating state of charge (SOC) meter exists, for example, which integrates the power stored or discharged by the battery, compares it with the previously known electrical cal capacity of the battery, and thereby measures the SOC. Using this type of SOC meter there is no need to modify the battery in any way, and as the SOC meter is reset when the battery is full due to regular charging, a relatively accurate SOC measurement can be made.
This SOC meter however assumes that the capacity of the battery when fully charged does not change, and if this capacity does change, the measurement accuracy becomes poorer. In particular, if the SOC is low, i.e. if much of the battery power has been discharged, the precision of the residual capacity obtained by subtracting the electrical capacity when the battery has discharged from the capacity when the battery is full, is no longer good.
In order to solve this problem, there are methods for determining residual capacity such as is disclosed, for example, in Japanese Tokkai Sho 63-157078. This method measures the capacity remaining in a common lead storage battery from the current and voltage when the battery is discharging a high current. The method is moreover based on the fact that the residual capacity of a lead battery has a certain correlation with the differential internal resistance of the battery for high current discharge (obtained as the linear slope of voltage vs. current for high discharge current). In other words, as disclosed in Tokkai Sho 63-157078, if the value of the current is fixed when a high current is being discharged (discharge current 200A), there is a good correlation between battery voltage and residual capacity. If the correlation between discharge voltage and residual capacity is found for different discharge voltages, the residual battery capacity can be found from the current and voltage for a high discharge current. Using this method, the residual battery capacity can be measured without modifying the battery in any way simply by measuring current and voltage. It is therefore regarded as suitable for measuring the residual battery capacity in an electric car.
The aforesaid conventional method can however only be applied when the discharge current is large. In other words, when the current is small, the relation between voltage/current characteristics and residual battery capacity fluctuates considerably and an accurate measurement of residual capacity cannot be obtained. This is considered to be due to the fact that immediately after battery charging or after prolonged discharge, the internal state of the battery is not the same, and for small current values, the voltage/current characteristics change as a result of the internal state of the battery.
For this reason, the voltage and discharge current are measured under conditions of high discharge current, and to ensure that a high discharge current is obtained, a separate discharge device must be connected so as to pass a large current. This discharge device necessarily has a large size, and as the battery capacity is reduced by this discharge, the running distance of the car for one charge is also reduced.
Alternatively, in Tokkai Hei 1-229986, a method is disclosed for measuring the residual capacity in an auxiliary battery of an engine-driven car from the current and voltage when the engine starts. As this type of method makes use of the fact that a high current flows when the engine starts, it can be applied only to an engine-driven car, and could not be applied to measure the residual battery capacity in an electric car.
Although for example a lead battery can normally be charged and discharged about 1000 times, the battery deteriorates by repeating the charging, and the full charge capacity decreases. In order to suitably measure the residual battery capacity, therefore, it is necessary to detect its degree of deterioration.
Tokkai Sho 63-261179 relates to an auxiliary battery of a car, and it also shows a device for detecting the degree of deterioration of the battery. Using this device, the differential internal resistance of the battery is found from the battery voltage when the battery is discharging a high current. As there is a good correlation between this differential internal resistance and the residual battery capacity, the residual capacity can be found from the measured differential internal resistance. The average current when the set voltage at which the voltage regulator operates is reached, is also found. The average current when this set voltage is reached is smaller the higher the full charge capacity, and the higher the residual capacity. By determining the full charge capacity from the residual capacity as found from the differential internal resistance and from the average current when the set voltage is reached, the degree of deterioration of the battery may be detected by comparing it with the rated full charge capacity.
If the degree of deterioration is thus determined, accurate values for SOC and residual capacity can be detected. In ordinary cases, the residual capacity is expressed by the electrical capacity Ah, and SOC is expressed as the residual capacity percentage of the full charge capacity. In detecting battery deterioration by the aforesaid prior art, the average charging current must be measured at the set voltage at which the voltage regulator operates. In the auxiliary battery of a gasoline engine, as the battery is always charging when the car is running, a situation often arises where the voltage regulator operates. In an electric car, however, the battery is mainly discharging when the car is running, and the average charging current, not in the highly charged state but when the set voltage has been reached, cannot be measured. Further, when the battery is being charged, the voltage is constant only in the final stage of charging, and it is impossible to accurately know the residual capacity at that time.
In the aforesaid example of the prior art, therefore, the degree of deterioration of the main battery used to drive the motor of an electric car could not be detected. It was also impossible to detect the precise value of the residual battery capacity taking account of the degree of deterioration.